The most easy way is to take airplane from Xian to Zhangjiajie directly, for there is no direct train or bus from Xian to Zhangjiajie. You can refer to this China Flight(https://www.travelchinaguide.com/china-flights/) for air tickets in a fixed day. Zhangjiajie Bus Station has direct bus to Wulingyuan, about 40-50 minutes. Then Zhangjiajie has direct train back to Shanghai South which takes 21 hours or so. Another option is to take direct bus from Zhangjiajie to Wuhan, then take bullet train to Shanghai then. It will be faster by this way.